Morphology Of Primary Particles Research Articles

Comments on “Microstructural and mechanical behavior of Mg/Mg2Si composite fabricated by a directional solidification system” by Mirshahi et al. [Mater. Sci. Eng. A 528 (2011) 8319–8323

Abstract This paper comments on a series of papers published in materials journals. These papers explain the effects of cooling rate and chemical modification on the microstructure and mechanical behavior of an in situ Mg/Mg2Si composite. The issues discussed here include the size and morphology of primary Mg2Si particles in the composite. The purpose of the present comment is to show that the sizes reported for primary Mg2Si particles are not reliable largely underestimating the correct values. The paper also includes criticisms of other mistakes in these papers.

Effect of the primary particle morphology on the micromechanical properties of nanostructured alumina agglomerates

Depending on the application of nanoparticles, certain characteristics of the product quality such as size, morphology, abrasion resistance, specific surface, dispersibility and tendency to agglomeration are important. These characteristics are a function of the physicochemical properties, i.e. the micromechanical properties of the nanostructured material. The micromechanical properties of these nanostructured agglomerates such as the maximum indentation force, the plastic and elastic deformation energy and the strength give information on the product properties, e.g. the efficiency of a dispersion process of the agglomerates, and can be measured by nanoindentation. In this study a Berkovich indenter tip was used for the characterisation of model aggregates out of sol–gel produced silica and precipitated alumina agglomerates with different primary particle morphologies (dimension of 15–40 nm). In general, the effect of the primary particle morphology and the presence or absence of solid bonds can be characterised by the measurement of the micromechanical properties via nanoindentation. The micromechanical behaviour of aggregates containing solid bonds is strongly affected by the elastic–plastic deformation behaviour of the solid bonds and the breakage of solid bonds. Moreover, varying the primary particle morphology for similar particle material and approximately isotropic agglomerate behaviour the particle–particle interactions within the agglomerates can be described by the elementar breaking stress according to the formula of Rumpf.

High Temperature CaO/Y2O3 Carbon Dioxide Absorbent with Enhanced Stability for Sorption-Enhanced Reforming Applications

To improve the stability of high temperature CO2 absorbent for sorption enhanced reforming applications yttria supported CaO were synthesized using two methods: calcination of mixed salt precursors and wet impregnation of yttria support. According to XRD data, CaO does not interact with the yttria matrix. However, introduction of CaO drastically changes the morphology of primary yttria particles. Increase in CaO concentration results in gradual plugging of the smaller pores and sintering of yttria support. The CO2 absorption uptake in recarbonation-decomposition cycles increases with increase in CaO content and reach 9.6 wt % at CaO content of 19.9 wt %. CaO recarbonation extent varies from 49 to 77%. CaO/Y2O3 absorbents are extremely stable under overheating and maintain their capacity in long series of decomposition-recarbonation cycles even after calcination at 1350 °C. The novel material resists moisture and retains its strength during storage in the air. According to tests, CaO/Y2O3 can be considered as a promising CO2 absorbent for fixed bed sorption enhanced hydrocarbons reforming.

The microstructure, hardness and tensile properties of Al–15%Mg 2Si in situ composite with yttrium addition

Current study investigates the effect of different concentrations of yttrium (0.1–1.0 wt.%) on the microstructure, hardness and tensile properties of Al–15%Mg 2Si cast composite. The microstructural study of the composite revealed the presence of both primary and secondary Mg 2Si phases in all specimens and also Y-containing intermetallics at higher concentrations of the respected element. It was also found that Y addition does not change the size and morphology of primary Mg 2Si particles considerably, but the pseudo-eutectic Mg 2Si was changed from a flake-like morphology to fine fibrous or rod-like. The results obtained from mechanical testing demonstrated that the addition of Y increases both hardness and ultimate tensile strength (UTS) values. Further investigations on tensile test revealed optimum Y level (0.5 wt.%) for improving both UTS and elongation values. Fracture surfaces via scanning electron microscopy (SEM) revealed that all specimens with large facets of primary Mg 2Si particles succumb to brittle fracture. The large and brittle Mg 2Si phases may act as crack initiators, while fine rod-like morphology of eutectic Mg 2Si in Al–Mg 2Si–0.5Y can be a barrier to the propagation of cracks and thus enhance the elongation values. At higher Y contents, an intermetallic phase (Al 2Y) introduced on eutectic cell boundaries, appears to be the favored path for crack propagation.

Morphology and Microstructure of Li/MgO Catalysts for the Oxidative Coupling of Methane

AbstractA series of catalysts for the oxidative coupling of methane (OCM) based on MgO with a varying content of Li have been synthesized by the gel‐combustion method. The resulting catalytically active systems are studied by a combination of TEM and SEM methods. Samples with a low abundance of Li exhibit a hierarchical pore system built from tubular structures made from primary MgO particles. Upon calcination at 1073 K, these particles undergo a change in shape from cubic via truncated octahedral to platelet morphologies, depending on the Li content of the precursor. Morphological indications have been found for the role of Li as flux in this transformation. The modification of the primary particle morphology leads to a drastic change in secondary structure from open sponges to compact sintered plates upon addition of Li at loadings above 10 wt %, with respect to the precursor. The microstructure of the primary particles reveals two families of high‐energy structures, namely edge‐and‐step structures and protrusions on flat terraces. A relation was found between catalytic function in OCM and the transformation from cubic to complex‐ terminated particles. Based on these findings, it is suggested that sites active for the coupling reaction of methane are related to the protrusions arising from segregation of oxygen vacancies to the surface of MgO.

Preparation of Titania with Different Morphologies

In this study, the commercial titanium dioxide (P25) was used to prepare titania aggregates with different morphologies The preparation procedure included the alkali treatment, hydrothermal with different reaction temperature and time, and sequentially spray drying. The results show that the hydrothermal temperature and time have a great influence on the morphology of primary titania particles. The results show that the one-dimensional nanotube, nanorod, and nanowire titania are obtained at T=140°C for t=24h, T=180°C for t=24h, and T=180°C for t=72h, respectively. Moreover, the obtained titania with different primary shapes are successfully applied to prepare the titania secondary particles with various morphologies, i.e., ring-like net structure, bird nest-like porous structure, and dense spherical structure. The optical absorption spectra also show that the titania aggregates obtained after spray drying have a higher optical absorbance in a range of ultraviolet and visible regions.

Development and characterization of a Versatile Engineered Nanomaterial Generation System (VENGES) suitable for toxicological studies

A novel system for generation of engineered nanomaterials (ENMs) suitable for in situ toxicological characterization within biological matrices was developed. This Versatile Engineered Nanomaterial Generation System (VENGES) is based on industry-relevant, flame spray pyrolysis aerosol reactors that can scaleably produce ENMs with controlled primary and aggregate particle size, crystallinity, and morphology. ENMs are produced continuously in the gas phase, allowing their continuous transfer to inhalation chambers, without altering their state of agglomeration. Freshly generated ENMs are also collected on Teflon filters for subsequent physicochemical and morphological characterization and for in vitro toxicological studies. The ability of the VENGES system to generate families of ENMs of pure and selected mixtures of iron oxide, silica, and nanosilver with controlled physicochemical properties was demonstrated using a range of state-of-the-art-techniques. Specific surface area was measured by nitrogen adsorption using the Brunauer–Emmett–Teller method, and crystallinity was characterized by X-ray diffraction. Particle morphology and size were evaluated by scanning and transmission electron microscopy. The suitability of the VENGES system for toxicological studies was also shown in both in vivo and in vitro studies involving Sprague–Dawley rats and human alveolar-like monocyte derived macrophages, respectively. We demonstrated linkage between physicochemical ENM properties and potential toxicity.

Effect of Primary Particle Morphology on the Structure of Gels Formed in Intense Turbulent Shear

We study the effect of primary particle morphology on intense shear-induced gelation without adding electrolytes. The primary particles are composed of a rubbery core grafted with a polystyrene shell. Depending on the shell-to-core mass ratio, the core can be partially covered by the shell, leading to strawberry-like morphology. It is found that at a fixed core mass the fractal dimension of the clusters constructing the gel decreases (i.e., more open cluster structure) as the shell mass increases, until reaching a plateau. The SEM pictures of the gels reveal that the structure variations are due to the occurrence of partial coalescence among particles, which decreases as the shell mass increases. In the region where the fractal dimension reaches a plateau, the coalescence is negligible. The conversion of the primary particles to gels is incomplete and increases as the extent of coalescence decreases. This is related to the fact that the smaller the extent of coalescence, the larger the cluster size. Thus, because of its cubic dependence on the cluster size, the aggregation rate increases as the extent of coalescence decreases, leading to increased conversion. It is therefore evident that the key parameter controlling the gel structure and the particle conversion is the core surface coverage by the shell. To further verify this conclusion, we have carried out the shear-induced gelation of another set of particles with varying core mass. It is found that the only parameter that can well correlate the values of the fractal dimension and particle conversion from the two sets of particles is the core surface coverage.

Morphology of mixed primary and secondary organic particles and the adsorption of spectator organic gases during aerosol formation

Primary organic aerosol (POA) and associated vapors can play an important role in determining the formation and properties of secondary organic aerosol (SOA). If SOA and POA are miscible, POA will significantly enhance SOA formation and some POA vapor will incorporate into SOA particles. When the two are not miscible, condensation of SOA on POA particles forms particles with complex morphology. In addition, POA vapor can adsorb to the surface of SOA particles increasing their mass and affecting their evaporation rates. To gain insight into SOA/POA interactions we present a detailed experimental investigation of the morphologies of SOA particles formed during ozonolysis of alpha-pinene in the presence of dioctyl phthalate (DOP) particles, serving as a simplified model of hydrophobic POA, using a single-particle mass spectrometer. Ultraviolet laser depth-profiling experiments were used to characterize two different types of mixed SOA/DOP particles: those formed by condensation of the oxidized alpha-pinene products on size-selected DOP particles and by condensation of DOP on size-selected alpha-pinene SOA particles. The results show that the hydrophilic SOA and hydrophobic DOP do not mix but instead form layered phases. In addition, an examination of homogeneously nucleated SOA particles formed in the presence of DOP vapor shows them to have an adsorbed DOP coating layer that is approximately 4 nm thick and carries 12% of the particles mass. These results may have implications for SOA formation and behavior in the atmosphere, where numerous organic compounds with various volatilities and different polarities are present.

Polysaccharides, Proteins, and Phytoplankton Fragments: Four Chemically Distinct Types of Marine Primary Organic Aerosol Classified by Single Particle Spectromicroscopy

Carbon‐containing aerosol particles collected in the Arctic and southeastern Pacific marine boundary layers show distinct chemical signatures of proteins, calcareous phytoplankton, and two types of polysaccharides in Near‐Edge Absorption X‐ray Fine Structure (NEXAFS) spectromicroscopy. Arctic samples contained mostly supermicron sea salt cuboids with a polysaccharide‐like organic coating. Southeastern Pacific samples contained both continental and marine aerosol types; of the 28 analyzed marine particles, 19 were characterized by sharp alkane and inorganic carbonate peaks in NEXAFS spectra and are identified as fragments of calcareous phytoplankton. Submicron spherical particles with spectral similarities to carbohydrate‐like marine sediments were also observed in Pacific samples. In both regions, supermicron amide and alkane‐containing particles resembling marine proteinaceous material were observed. These four chemical types provide a framework that incorporates several independent reports of previous marine aerosol observations, showing the diversity of the composition and morphology of ocean‐derived primary particles.

Alkyl sulfonate modified LDH: Effect of alkyl chain length on intercalation behavior, particle morphology and thermal stability

Mg–Al layered double hydroxide (LDH) was modified with straight-chain primary alkyl sulfonates by regeneration. The effect of alkyl chain length on the intercalation behavior of LDH, particle morphology and thermal stability of the modified LDH were investigated in detail. Irrespective of the chain length, all the alkyl sulfonate was very efficiently intercalated. The alkyl sulfonates usually form monolayers in the interlayer space, except the C18 alkyl sulfonate. The organic modification via regeneration not only changed the primary particle morphology, but at the same time size of alkyl chain significantly influenced the crystal growth during the regeneration. The intercalated alkyl sulfonates decomposed above 300 °C and the increasing hydrocarbon chain decreased the decomposition temperature. The alkyl sulfonates stabilized the metal hydroxide layers.

Microstructure evolution of A356 alloy in a novel rheocasting approach

A novel approach, was designed to obtain thixotropic structure of equiaxed or non-dendrtic grains for rheocasting, and demonstrated experimentally using A356 aluminum alloy. The circumstance in bulk liquid metal, which would burst into copious nucleation, and at the same time create homogeneous distribution of temperature and solute, was constructed by combined utilization of rotating magnetic fields and a cooling tube. Both the solidified microstructures in metal mold and sand mold exhibited non-dendritic characteristic. Analyses of the rheocasting A356 microstructure indicated that high density of nuclei occurred by inserting a cooling tube into rotating slurry at liquidus temperature. In the case of slow cooling rate, mushy slurry obtained with high nuclei density kept non-dendritic morphology of primary particles with holding time, accompanying grain particles’ coarsening and spheroidizing; moreover, the eutectic silicon got coarsening with retaining time.

Distribution and accessibility of binder in wet granules

The effect of primary particle morphology on the spatial distribution of binder in wet granules was investigated by numerical simulations. The shape factors of four commonly used pharmaceutical excipients – mannitol, lactose, microcrystalline cellulose, and calcium phosphate – were evaluated by digital image analysis and used for three-dimensional computer reconstruction of virtual particle populations. The formation of wet agglomerates was simulated by close random packing of primary particles and then finding the equilibrium distribution of a liquid binder in the pore space within the close packed structures, using the Volume of Fluid (VOF) method. The spatial distribution of binder in the computer-generated wet agglomerates was then analysed and the dependence of the fractional surface coverage by liquid on the overall binder/solids ratio was systematically obtained for different values of primary particle surface roughness. The obtained dependence was used to explain experimentally observed differences in the granulation kinetics of the four pharmaceutical excipients under otherwise identical conditions.

Characterization of TiO2 Particles Synthesized in Diffusion Flame Reactor

Nanoscale TiO2 particles were synthesized in the diffusion flame reactor from the titanium tetraisopropoxide (TTIP) decomposition reaction. The effects of O2 and CH4 flow rates and the filter position in the reactor on the specific surface area, primary and secondary particle sizes, morphology, phase composition, and crystallite size of TiO2 particles were investigated by Brunauer-Emmett-Teller (BET), electrophoretic light scattering (ELS), transmission electron microscopy (TEM), and X-ray diffraction (XRD) techniques. The larger primary particles can be observed at the lower O2 flow rate or the higher CH4 flow rate, while the secondary particle size increases at the higher O2 flow rate or the lower CH4 flow rate. The degree of aggregation increases as the TiO2 particles are collected in a higher position on the filter. The transformation of anatase to rutile phase and the growth of anatase grain size in the TiO2 particles are promoted as the flame temperature increases.

Characterization of components of nano-energetics by small-angle scattering techniques

Small-angle scattering (SAS) and ultra small-angle scattering techniques, employing x-rays and neutrons, were used to characterize six different aluminum nanopowders and nanopowders composed of molybdenum trioxide and tungsten trioxide nanoparticles. Each material has different primary particle morphology and aggregate and agglomerate geometry, and each is important to the development of nano-energetic materials. The combination of small-angle and ultra small-angle techniques allowed a wide range of length scales to be probed, providing a more complete characterization of the materials. For the aluminum-based materials, differences in the scattering of x-rays and neutrons from aluminum and aluminum oxide provided sensitivity to the metal core and metal oxide shell structure of the primary nanoparticles. Small-angle scattering was able to discriminate between particle size and shape and agglomerate and aggregate geometry, allowing analysis of both aspects of the structure. Using the results of these analyses and guided by scanning electron microscopy (SEM) images, physical models were developed, allowing for a quantitative determination of particle morphology, mean nanoparticle size, nanoparticle size distribution, surface layer thickness, and aggregate and agglomerate fractal dimension. Particle size distributions calculated using a maximum entropy algorithm or by assuming a log-normal particle size distribution function were comparable. Surface area and density determinations from the small-angle scattering measurements were comparable to those obtained from other, more commonly used analytical techniques: gas sorption using Brunauer–Emmett–Teller analysis, thermogravimetric analysis, and helium pycnometry. Particle size distribution functions derived from the SAS measurements agreed well with those obtained from SEM.

Refinement of primary Si in hypereutectic Al–Si alloy by electromagnetic stirring

Abstract The effect of electromagnetic stirring on the morphology of primary Si particles in a hypereutectic aluminum–silicon alloy was investigated in the semi-solid temperature region. It was found that with the stirring current decreasing from 32 to 16 A, the primary Si particles congregated severely, formed large primary Si particles. When the current was less than 12 A, however, the particle distribution became uniform and there was no any particle congregation. The primary Si particles reached a minimum equivalent diameter of 44.8 μm when the stirring current was 8 A. Nevertheless, when the stirring current was less than 8 A, the primary Si particles coarsened again gradually. The congregating of primary Si particles is attributed to the presence of centrifugal force field caused by electromagnetic stirring, which forced the Si particles to move in radial direction, distribute in grade and congregate. The diameter of the Si particles is decided by the cooperation of the congregation effect and refining effect of electromagnetic stirring. The present research shows that a relatively weak electromagnetic stirring is more effective in refining the primary Si particles than a strong electromagnetic stirring.

The microstructural characterization of semi-solid slurries

Due to recent interest in semi-solid metal (SSM) processing, there is a need for fundamental knowledge on the formation and solidification of primary particles. The primary particle size, distribution, morphology, and percentage are the main concerns because particle quantity and size affect not only the mechanical properties of as-cast SSM parts but also the flow characteristics of SSM slurries during die filling. Microstructural characterization is a basic tool for measuring the critical parameters that influence the resulting properties. This article describes the tools and techniques available for structural analysis of SSM slurries. It also attempts to elucidate the ambiguities and interpretations of special structural features.

Effect of holding temperature on semisolid microstructure of Mg 2Si/Al composite

A semisolid microstructure of Mg 2Si/Al composite is successfully achieved by isothermal heat treatment process, and the effects of holding temperature on semisolid microstructure of the composites are investigated. The result shows that, at the low heat treatment temperature, the morphology of primary Mg 2Si particles are identified as irregular shape and α-Al grains exhibit elliptic and/or rosette type, and their sizes are non-uniform from 35 to 130 μm and 41 to 191 μm, respectively. With the increasing of treatment temperature, the morphologies of primary Mg 2Si and α-Al grains change to spherical type, and their sizes decrease and become relatively uniform.

Influence of Silicon Morphology and Mechanical Properties of Piston Alloys

In this paper, the influence of silicon particle morphology on mechanical properties of two piston alloys has been reported. Alloys having nominal composition Al-12%Si-1%Ni-0.8%Cu-0.6%Mg (LM13) and Al-17%Si-1%Ni-0.8%Cu-0.6%Mg (LM28) were prepared by melting and casting. The morphology of silicon crystals was changed using additives during the melt treatment and subsequently by heat treatment. Mechanical properties such as hardness, ductility, and tensile strength of experimental alloys were tested. Test results showed that the melt treatment and heat treatment of both the alloys increased the tensile strength, hardness, and ductility. However, the influence of melt treatment on mechanical properties was not as significant as that of heat treatment. Tensile strength and ductility of LM13 was found to be higher than the LM28 alloy under similar conditions. Hardness of LM28 was higher than the LM13 alloy. Optical and SEM studies showed a change in eutectic silicon morphology on melt treatment of LM13. However, change in morphology of primary silicon particles was not significant as for LM28. Heat treatment of both alloys caused spheroidization and better distribution of eutectic silicon crystals. Sharp-edged primary silicon particles were rounded off after the heat treatment of LM28. Scanning electron microscopy (SEM) of tensile fractured surfaces was carried out to study the influence of microstructure on fracture mode. Heat treatment of both alloys promotes dimple fracture. However, as-cast and melt-treated alloys show predominantly cleavage fracture.

Computer simulation of granule microstructure formation

The diagenesis (porous microstructure evolution) of granules formed by a layering growth mechanism in a wet granulation process has been modelled. The model includes the packing of primary particles with a given size and shape distribution, and the deposition, spreading, and solidification of binder droplets within the growing granule. The dependence of granule porosity on the binder/solids ratio, primary particle size and morphology, and the rates of binder spreading and solidification has been investigated. The results are presented in the form of structure maps relating volume-averaged microstructure parameters with dimensionless groups including the ratio of droplet spreading and solidification times and the mean time between particle collisions. These graphs can guide the selection of process operating conditions or formulation ingredient properties required to obtain a particular granule microstructure.